The present invention relates to a printing machine, and more particularly to a printing machine including a sheet transfer roller which transfers sheets between the printing rollers of an upstream and a downstream printing station.
It is known to provide printing machines having two or more printing stations, with sheet transfer rollers which transfer sheets from a printing roller of an upstream printing station to the printing roller of the next or downstream printing station. German Pat. No. 81,406 teaches a construction in which the sheet on the printing roller of the upstream printing station is engaged at the tangent point defined between this roller and the sheet transfer roller, by a suction gripper system which is movable in direction of the periphery of the transfer roller, and which tensions the sheet. After the tangent point is left, a first mechanical gripper system engages the sheet, taking it over from the suction gripper system at the periphery of the sheet transfer roller. Thereupon, the first gripper system tilts or pivots inwardly of the periphery of the sheet transfer roller and yields the sheet to the second mechanical gripper system which then tilts outwardly and hands the sheet over to a gripper on the printing roller of the downstream printing station.
This construction is generally satisfactory up to a certain printing speed. However, when certain types of sheets are being printed, and when the printing speed amounts to values on the order of 8,000 sheets per hour, deviations may occur in the position of the sheets from the desired position due to the fact that the suction gripper system must pull the sheet so as to cause it to perform a movement relative to the direction of rotation of the printing cylinder from which it takes over the sheet.
Furthermore, since the sheet is guided by the suction gripper system, once it has been engaged by the same, to a zone outside the tangent point of the first printing roller and the sheet transfer roller, the so-called tilting angle (i.e. the angle between the suction surface of the suction gripper system and the remainder of the sheet which is still on the periphery of the first printing roller) is very large, so that if the sheet is of a relatively stiff material and if the machine operates at high speed, the sheet may flex off the suction surface of the suction gripper system, so that it will not be in its desired position at the time the first mechanical gripper system is ready to engage the sheet.